Ion beam and plasma processing of workpieces (substrates) may be performed for a variety of purposes including for ion implantation, surface texturing, and etching of a surface. Ion implantation in particular is a standard technique for introducing property-altering impurities into substrates. A desired impurity material is ionized in an plasma source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the sub-surface of the substrate material and are embedded into the crystalline lattice of the substrate material to form a region of desired conductivity or material property.
One challenge for ion beam processing is the need to dissipate charge at a workpiece, which may occur during ion implantation of a workpiece because ions impinging on a substrate by nature carry charge. In the case of ion beams that comprise positive ions, positive charge may build up on the workpiece after exposure to an ion beam. In order for this charge to be dissipated, the workpiece holder may be grounded, thereby providing a conductive path for conducting the charge from the workpiece surface. However, if a workpiece itself is a poor conductor or an electrical insulator, the charge on the workpiece surface may have no conductive path to ground, thereby preventing the charge from being dissipated.
Neutralization of charge that builds up on a workpiece surface due to exposure to an ion beam may also be accomplished by providing charged species of opposite polarity to the charge on the workpiece. In typical known ion implantation systems that employ pulsed ion implantation using positive ions, including plasma immersion ion implantation, a plasma may be established proximate a workpiece holder and a periodic bias may be applied in pulses between the plasma and workpiece holder. During “on” periods positive ions may be attracted to the workpiece by providing a bias between the plasma and workpiece holder, wherein the potential at the workpiece holder is negative with respect to the plasma. At the same time, electrons in the plasma may be repelled from the workpiece holder due to its relatively negative potential with respect to the plasma. During “off” periods when the implantation system no longer sets the workpiece holder at a negative potential with respect to the plasma, electrons may drift towards the workpiece. However, the flux of electrons during these “off” periods may be insufficient to neutralize the surface of the workpiece and excessive positive charge may remain.
FIG. 1a illustrates a voltage pulse train 100 that includes a series of “on” periods 102 interrupted by “off” periods 104. During the “on” periods 102 a positive high voltage may be applied to a plasma source, while the workpiece is grounded, thereby setting the workpiece at a high negative potential (voltage) with respect to the plasma. Accordingly, positive ions may be attracted to the workpiece at a high energy of about 10 kV in the example shown in FIG. 1a. During the “off” periods 104, when the DC voltage of the plasma source is nominally at ground potential, in principle the voltage between plasma and workpiece is about zero. Accordingly some electrons may drift out of a plasma and towards a workpiece during the “off” periods 104, thereby tending to neutralize the workpiece.
FIG. 2 provides an illustration of circuitry 202 that may be used to create the voltage pulse train 100. As depicted in FIG. 2, a plasma source 210 is coupled to the circuitry 202 to provide pulsed ion beams 214 to workpiece holder 212. The circuitry 202 includes a high voltage power supply 204, and a high voltage switch 206 that can connect or disconnect the high voltage power supply 204 to the plasma source 210. When a plasma is created in the plasma source 210, the plasma source 210 may be biased to a high positive potential, such as +10 kV illustrated in FIG. 1a, by closing high voltage switch 206. This high positive potential serves to extract positive ions 214 from plasma source 210 and accelerate the positive ions 214 toward workpiece holder. When high voltage switch 206 is open and second switch 208 is closed, the plasma source is grounded and positive ions are no longer attracted toward workpiece holder 212. Accordingly, by alternating between configurations in which one switch of switches 206, 208 is open, and the other closed, pulsed ion beams 214 may be created during “on” periods 202.
While the circuitry 202 may produce a waveform generally as shown by voltage pulse train, an actual voltage waveform may differ from a desired waveform where the voltage is zero during “off” periods. For example, the high voltage switch 206 and second switch 208 may have an internal impedance that results in a small voltage drop. Thus, during the “off” periods 104 in which the plasma source is connected through second switch 208 to ground, the small internal impedance of second switch 208 may result in plasma source 210 not being directly grounded, but rather floating at a potential that may be several volts above zero. As illustrated in FIG. 1b, which shows a more expanded view of one “off” period 104, the plasma source may actually acquire a potential up to several volts positive due to the internal impedance. During the “off” periods 104, the resulting positive bias of plasma source 210 with respect to workpiece holder 212 impedes the flow of electrons to workpiece holder 212, since the workpiece holder potential is several volts more negative than the plasma source potential. Thus, during these “off” periods the flux of electrons from the plasma with sufficient initial energy to overcome the negative potential of the workpiece may be insufficient to neutralize the surface of the workpiece, such that excessive positive charge may remain.
In view of the above, it will be appreciated that it may be useful to provide improvements for neutralization of charge in systems that provide charged species of a predominant polarity, such as ion beam systems.